1. Field of the Invention
This invention is concerned with an improved process for the preparation of esters by the reaction of acyl halides and alcohols or by the reaction of hydroxyalkyl esters with acid halides of acids wherein the reactions are carried out in the presence of a tertiary amine as an acceptor for the hydrogen halide generated in the reaction. The invention is especially concerned with an improved process of this type for the preparation of bis-acrylates from an acryl chloride and a diol, or a hydroxyalkyl acrylate and a dicarboxylic acid chloride.
2. Description of the Prior Art
In the past, many esters have routinely been prepared by the reaction of an acyl halide and an alcohol in a non-polar solvent such as toluene or chloroform using a tertiary amine as an acceptor for the hydrogen halide generated in the reaction. Such esters have also been prepared by the reaction of hydroxyalkyl esters with acid halides of carboxylic acids. By way of exemplification, bisacrylates have usually been prepared by the reaction of an acryl chloride and a diol in a non-polar solvent using a tertiary amine such as triethylamine as an acceptor for the hydrogen chloride generated in the reaction. In a similar manner bisacrylates have also been prepared by the reaction of hydroxyalkyl acrylates with acid chlorides of dicarboxylic acids. While these reactions have been routinely carried out in non-polar solvents using tertiary amines as acceptors for the hydrogen halide generated in the reaction, a number of drawbacks are presented, however, when it is attemped to recover the end products from the reaction mixture. In order to remove the amine hydrohalide which is formed, the reaction mixture in the past has been washed with water, and then treated with dilute acid to remove excess amine. It has often happened in the past that with the addition of the water or the dilute acid, emulsions were formed which were very hard to break. In many instances, the desired products were degraded by hydrolysis or undesired polymerization occurred before the emulsion could be broken.
Examples of compounds which have been prepared by these reactions in the past are monomeric bisacrylic resins such as the following: ##STR1## wherein R.sub.1, R.sub.2 and R.sub.3 are either hydrogen or lower alkyl groups preferably comprising 4 or less carbon atoms, especially the compounds bis-phenol-A-bis(2-methacrylatoethyl) ether which is represented by the formula: ##STR2## bisphenol-F-bis(2-methacrylatopropyl) ether which is represented by the following formula: ##STR3## bis(2-methacrylatoethyl) phthalate which is represented by the following formula: ##STR4## bis(2-methacrylatoethyl) isophthalate which is represented by the following formula: ##STR5## bis(2-methacrylatoethyl) terephthalate which is represented by the following formula: ##STR6## and the bismethacrylate ester of bisphenol-A which is represented by the following formula: ##STR7##
The foregoing monomer compounds may be polymerized to form useful coatings and other protective finishes. They have also been found useful in dental restorative compositions when blended with inorganic fillers.
While it is known to prepare esters by the relatively simple reactions outlined above, for example, in the case of bisacrylates by the reaction of the acryl chloride with a diol or a hydroxyalkyl acrylate and the acid chloride of a dicarboxylic acid, in each instance, in the presence of a tertiary amine serving as an acceptor for the hydrogen chloride generated, the recovery of the end products from the reaction mixtures of these reactions has suffered from deficiencies caused by the formation of relatively stable emulsions when the reaction mixture was washed with water in order to remove the amine hydrochloride or washed with dilute acid to remove excess amine. The emulsions formed were generally hard to break and made recovery of the desired end product difficult. For example, in the case of the acrylate type of products, they were often partially degraded by hydrolysis or undesired polymerization occurred before the emulsion could be broken.